Field of the Invention
The invention relates to a process for producing tungsten silicide layers and to a semiconductor component which comprises tungsten silicide layers of this type.
Tungsten silicide has hitherto been used as a filling material for contact holes and as a gate material for transistors in microchips. The tungsten silicide is deposited on a substrate, generally a silicon wafer, using a CVD (chemical vapor deposition) process. The deposition takes place at temperatures in the range from 500 to 600xc2x0 C. at a chamber pressure of approximately 1 torr. Tungsten hexafluoride and dichlorosilane (DCS) are used as gaseous precursor substances. The WF6:DCS ratio is selected in the range of 1:200. As a result of reaction between the precursor substances, surface layers are deposited during the production of gate electrodes, and these layers have a uniform composition considered over their volume. Even during the filling of contact holes which have a low aspect ratio, no or only slight fluctuations in the composition of the WSix are observed over the volume. An aspect ratio is understood as meaning the ratio of the depth of a recess to the diameter of its openings. If the known process is used to deposit WSix layers for filling recesses with a high aspect ratio, as is the case, for example, in the production of deep trench capacitors in DRAM memory modules, only very poor filling of the recess is achieved. During the deposition, the opening of the recess quickly becomes closed up and prevents the precursor substances from penetrating into the lower-regions of the recess. Furthermore, a considerable change in the stoichiometry of the tungsten silicide is observed during the deposition. Therefore, the composition of the tungsten silicide changes over the depth of the recess. The tungsten silicide is therefore not in a thermodynamic stable state. The result of this is that during completion of the semiconductor module in subsequent conditioning steps, either, in the case of a high tungsten content in the tungsten silicide, silicon migrates out of the substrate into the tungsten silicide layer or, in the case of a high silicon content in the tungsten silicide, silicon is separated out of the tungsten silicide. Both reactions lead to a deterioration in the properties of the semiconductor module.
It is accordingly an object of the invention to provide a method of depositing tungsten silicide layers which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which allows deposition of tungsten silicide even in recesses with a high aspect ratio, while the stoichiometry exhibits only minor fluctuations over the volume of the tungsten silicide layer that has been deposited.
With the foregoing and other objects in view there is provided, in accordance with the invention, a process for producing a tungsten silicide layer, which comprises:
depositing a tungsten silicide layer on a substrate at a temperature of less than 400xc2x0 C. and a pressure of less than 50 torr (preferably  less than 20 torr, and specifically  less than 10 torr) from a vapor phase with a tungsten-containing precursor substance and a silicon-containing precursor substance; and
setting a molar ratio of silicon-containing precursor compound to tungsten-containing precursor compound in the vapor phase to greater than 500.
The process according to the invention is carried out at temperatures which are significantly lower than in the processes which are known from the prior art. Furthermore, the molar ratio of silicon-containing precursor substance to tungsten-containing precursor substance is selected to be significantly higher than in the known process. These measures make it possible to produce tungsten silicide layers with a high edge coverage even in recesses with a high aspect ratio. The term edge coverage is understood as meaning the ratio of the layer thickness of the tungsten silicide at the surface of the substrate to the layer thickness of the tungsten silicide at the lowest point in the recess. Furthermore, the tungsten silicide layers which have been deposited using the processes according to the invention have only slight fluctuations in the stoichiometry even in the case of structural elements which have a considerable depth in the wafer. This makes the tungsten silicide layers produced thermally stable, i.e. there are no changes in the composition of the tungsten silicide layer produced even during subsequent process steps wherein the wafer is conditioned at elevated temperature.
The inventors explain this effect by means of the mechanism which is outlined below. However, it should be understood that the mechanism is merely a theory and is not intended to restrict the scope of the invention. Substantially the three reactions which are outlined below take place during the deposition of a tungsten silicide layer:
SiH2Cl2xe2x86x92Si5+2HClxe2x80x83xe2x80x83(I) 
WF6+4SiH2Cl2xe2x86x92WSi25+8HCl+SiF4+SiF2xe2x80x83xe2x80x83(II) 
5WF6+11SiH2Cl2xe2x86x92W5Si35+22HCl+7SiF4+SiF2xe2x80x83xe2x80x83(III) 
In the processes which are known from the prior art, the deposition of the tungsten silicide takes place substantially in accordance with the reaction which is presented by equation (II). The deposition of the tungsten silicide takes place very quickly, so that the reaction takes place substantially in a diffusion-controlled manner. This means that, in the case of recesses with a high aspect ratio, the deposition of the tungsten silicide takes place substantially close to the upper opening of the recess, with the result that the latter quickly closes up, and only small quantities are deposited in the lower regions of the recess. If the deposition temperature is reduced, in order in this way to reduce the reaction rate, the relative share formed by the reaction presented by equation (III) rises. Reaction III has the lowest activation energy compared to the other reactions presented. This means that now, although an improved edge coverage is achieved, i.e. a tungsten silicide layer is produced even in the lower regions of the recess, at the same time the stoichiometry of the tungsten silicide layer shifts in favor of the tungsten component. This means that in subsequent conditioning steps silicon migrates from the wafer into the deposited tungsten silicide in order to achieve a thermodynamically stable state. This shift in the stoichiometry when using slow deposition of the tungsten silicide layer is now compensated for, according to the invention, by drastically increasing the proportion of the silicon-containing precursor compound in the vapor phase compared to the processes which are known from the prior art. As a result, there is an increased deposition of silicon at the low deposition temperature, resulting from a reaction presented by equation (I). In this way, it is possible to compensate for the increased tungsten content, so that a thermally stable tungsten silicide layer is obtained. The two measures according to the invention, (a) reducing the deposition temperature, and (b) increasing the proportion of the silicon-containing precursor substance in the vapor phase, mean that with the processes according to the invention it is for the first time possible to produce tungsten silicide layers which have only slight fluctuations in the stoichiometry over their entire volume and which are thermodynamically stable in recesses with a high aspect ratio.
The tungsten silicide layer which is deposited using the process according to the invention preferably has a composition WSix wherein x is selected in the range from 2xe2x89xa6xxe2x89xa63. The composition of the tungsten silicide layer can be achieved by suitable selection of the deposition temperature and/or by changing the molar ratio of the gaseous precursor substances. The reaction parameters are dependent on the dimensioning and the aspect ratio of the recess wherein the tungsten silicide layer is to be deposited, and also on the equipment used for deposition. The precise reaction parameters can be determined by the person skilled in the art using suitable preliminary tests.
The tungsten-containing precursor compound used is preferably tungsten hexafluoride. This compound is already used for the production of tungsten silicide layers for gate electrodes, and consequently there is already experience of this compound for process management. The same applies to the silicon-containing precursor substance, for which it is preferable to use dichlorosilane. A further advantage is that in principle it is possible to use existing production facilities for the process according to the invention, and it is merely necessary to modify the reaction conditions for the deposition in accordance with the process according to the invention.
The deposition of the WSix from the vapor phase can be carried out in such a manner that no carrier gas is added to the vapor phase in the deposition device. However, in practice it has proven advantageous for a carrier gas to be added to the vapor phase. The carrier gas used may be a noble gas, preferably argon, or another inert gas, for example gaseous nitrogen. Since a very high ratio of silicon-containing precursor substance to tungsten-containing precursor substance is selected, it is very difficult to add the tungsten-containing precursor substance, since it is only supplied to the deposition device in very small quantities. Therefore, it is advantageous for the tungsten-containing precursor substance to be diluted with the carrier gas.
The substrate used is preferably a silicon substrate, a silicon wafer being particularly preferred. The process according to the invention can also be used for the production of planar layers on the surface of the silicon substrate. However, its particular benefits arise especially if the substrate includes recesses, in particular recesses with an aspect ratio of more than 20. It is then possible to produce uniform tungsten silicide layers even in the lower regions of the recesses, i.e. in the sections of the recess which are arranged inside the substrate.
As has already been stated above, the process according to the invention can produce thermodynamically stable tungsten silicide layers even in recesses with a high aspect ratio. This leads to semiconductor components with improved properties. Therefore, the invention also relates to a semiconductor component comprising a silicon substrate wherein deep trench capacitors are formed, which have a filling of WSix, the deep trench capacitors having an aspect ratio of more than 20, an edge coverage of more than 50% and furthermore x being greater than 2 over the volume of the WSix filling.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a process for depositing WSix layers on a high topography with a defined stoichiometry, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.